1. Technical Field
Embodiments disclosed herein relate generally to the field of borehole acoustic logging and more particularly to the field of dipole sources for acoustic logging in geophysical applications.
2. Description of Related Art
In the field of borehole acoustic logging, dipole sources generate borehole shear waves. Acoustic shear waves provide information related to the composition of a substrate surrounding a borehole. Acoustic dipole sources used currently include piezo-electric (PZ) plate bender bars, piston radiators, and shakers. Bender bars typically emit acoustic waves in a narrow frequency band between 1 kHz to 3 kHz (kilo-Hertz), limiting the measurement range of a logging device. Piston radiators provide a wider bandwidth, but have poor mode purity, typically including monopole, quadrupole and higher order acoustic modes associated with acoustic radiation. In addition, piston radiators lack very low frequency response, which is the soft formation spectral range of shear waves. Some attempts to create acoustic dipole sources have used shaker sources that produce shear waves transferred through the liquid in a borehole. However, shaker sources tend to induce interference of the driving mechanism into the logging tool body, thus requiring a good acoustic isolator to prevent interference. In addition, shaker sources can prove inadequate in providing strong dipole radiation, due to their mass.
In some applications, an explosive force increases the strength of an acoustic wave source. However, explosive forces are difficult to control and may result in multimode contamination of the acoustic signal.
What is needed is a dipole source for acoustic logging applications that has a wide bandwidth of operation and a high modal purity. In addition, what is needed is a dipole source for shear acoustic waves having low frequency and high signal strength.